import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.Queue;

/*二叉树的分层遍历*/
public class Test2 {
    public class TreeNode{
        int val;
        TreeNode left;
        TreeNode right;
        TreeNode(){}
        TreeNode(int val){
            this.val = val;
        }
    }
    public List levelOrder(TreeNode root){
        List<List<Integer>> list = new ArrayList<>();
        if (root == null){
            return list;
        }
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while (!queue.isEmpty()){
            int size = queue.size();
            List<Integer> tmp = new ArrayList<>();
            while (size != 0){
                TreeNode cur = queue.poll();
                tmp.add(cur.val);
                size--;
                if (cur.left != null){
                    queue.offer(cur.left);
                }
                if (cur.right != null){
                    queue.offer(cur.right);
                }
            }
            list.add(tmp);
        }
        return list;
    }
}
